The proposed study is designed to correlate host defense mechanism with occurrence of bacterial, fungal and viral nosocomial infections. Emphasis will be the application of techniques used to investigate antibody responses, cellular immunity, and polymorphonuclear leukocyte function in patients at risk of developing infection in the hospital. Patients will be studied when they have nosocomial infection, and following recovery. Prospectively, older patients, individuals with diabetes, recipients of renal allografts and patients admitted with solid tumors will be monitored after admission. The objective will be to determine if assessment of defense mechanisms can effectively predict which patients will be colonized and subsequently infected following admission to hospital.